This invention relates to a process for producing carbon fibers (graphite whiskers) More specifically, this invention relates to a process for producing graphite whiskers by the gas-phase thermal decomposition of a carbon source compound using as a catalyst ultrafine particles of a transition metal prepared in a specific manner.
As reported in "New Developments in Applied Carbon Techniques", CMC, page 13, Morimichi Endo, and "Catalytic Growth of Carbon Filaments", R.T.K. Baker, Carbon, Vol. 27, No. 3, 315 (1980), for instance, the gas-phase process for the production of graphite whiskers generally comprises supplying a gas of a carbon source compound together with ultrafine transition metal particles as a catalyst having a particle size of 0.5 to 0.001 .mu.m into a heating zone and decomposing the carbon source compound to produce carbon fibers in the form of whiskers.
Specific examples of such a process are disclosed in Japanese Patent Kokai (Laid-Open) Publication No. 52-103528, Japanese Patent Publication No. 53-7538, Japanese Patent Kokai Publication No. 57-117622, Japanese Patent Kokai Publication No. 61-136992 (EP-A 184317, U.S. Pat. No. 4663230) and so on. Japanese Patent Kokai Publication No. 61-136992, for instance, proposes a gas-phase process for producing carbon fibers using as a catalyst ultrafine iron particles which have been obtained by feeding a gas of Fe(CO).sub.5 into a reactor and decomposing the same.
Carbon fibers produced by these gas-phase processes are characterized by their higher crystallinity (graphitic nature) and taking a more whiskery morphology in comparison with carbon fibers usually based on PAN and pitch, and are thus expected to be applied to highly rigid structural materials, high-performance materials and so on, to which conventional carbon fibers have not yet been applied.
The properties of the carbon fibers produced by the gas-phase methods, e.g., their physicochemical properties such as crystallinity and rigidity as well as their morphological properties such as whiskers' length and thickness depend upon the reaction conditions, types of carbon sources and other factors. In particular, the length and thickness of whiskers correlate with, inter alia, the size and shape and these distribution of the catalyst particles used and so depend largely thereon.
In order to produce graphite whiskers of uniform length and thickness and having a sharp particle size distribution, it is required to use a catalyst of uniform particle size and shape as well as a sharp particle size distribution.
So far as the present inventors know, however, graphite whiskers obtained with the use of such a catalyst as disclosed in the above described prior art are so uneven in their length and thickness and so broad in their particle size distribution that there are problems with such graphite whiskers in their applications as described above. There is therefore left much to be desired in this respect.